1. Field of the Invention
The present invention relates to a neutron beam controlling apparatus that performs converging and diverging of a neutron beam, and a method for manufacturing the same.
2. Description of the Related Art
A neutron beam is different from an X ray or a photon, and has the following characteristics. (1) The neutron beam strongly interferes with an atomic nucleus. (2) Energy and a wavelength of the neutron beam have the same degree as motion and a structure in a level of an atom. (3) The neutron beam has a magnetic moment. (4) The neutron beam has strong penetration power. By taking advantage of such characteristics of the neutron beam, in a case of research of position itself of an atomic nucleus, for example, in a case of obtaining position information of a hydrogen atom in an organic material, a scattering experiment that uses a neutron beam is inevitable because it is extremely difficult to obtain such position information by X-ray scattering. Furthermore, since a neutron has a ½ spin and a magnetic moment, the neutron beam is useful for examining a magnetic structure of a material. Further, in a case where research of an inside of a large object such as an industrial product is performed by using radiation, the neutron beam having strong penetration power enables fluoroscoping of the large object.
However, it is not easy to generate a neutron beam, so that a place where the neutron beam can be available is limited to a nuclear reactor, an accelerator facility and the like. For this reason, in order that the neutron beam is efficiently introduced from the neutron source to an apparatus using the neutron beam, and a minute sample is irradiated with the high-density neutron beam, a beam controlling technique for raising a degree of parallelization of the neutron beams and focusing the neutron beams is inevitable.
Recently, attention has been paid to above-described analyzing that uses the neutron beam, and the applicant of present patent application has already proposed a device for converging and diverging of the neutron beams (refer to “Japanese Laid-Open Patent Publication No. 2001-062691). Hereinbelow, this device is referred to as a “neutron lens”).
FIG. 1 shows a principle of a refraction of a neutron beam by a substance. Almost interaction between a neutron and a substance is caused by interaction between the neutron and an atomic nucleus in the substance. Because of this interaction, when the incident neutron enters the inside of the substance, the neutron loses a part of its energy, so that the neutron is decelerated in the direction perpendicular to the boundary surface of the substance. Thereby, the neutron beam that obliquely enters the boundary surface of the substance is refracted such that a refractive index becomes a value smaller than 1 as shown in FIG. 1. A material that has a refractive index of less than 1 for a neutron beam includes oxygen O, carbon C, beryllium Be and fluorine F among those having naturally occurring isotopic concentrations, and deuterium D among enriched isotopes.
FIG. 2 shows a principle of a neutron lens, and shows a way in which a neutron beam 16 enters one plate member 11. Linear projections 12 each including an almost vertical surface 15 and an inclining surface 15 are formed on the plate member 11. The neutron beam 16 that enters the inclining surface 15 of the linear projection 12 is refracted such that a refractive index becomes lower than 1 similarly to FIG. 1. However, a refracted angle δ by this one refraction is minute. For example, when the plate member 11 is made of polytetrafluoroethylene (PTFE), and the inclining surface 15 of the linear projection 12 is inclined from a surface plane of the plate member 11 by an angle α of 45 degrees, an refracted angle of a neutron beam 16 that has a wavelength of 14 angstroms (Å) and vertically enters the plate member 11 is only 0.14 mrad.
FIG. 3 is a perspective view showing a neutron lens that has a function of focusing a neutron beam. FIG. 4 is a sectional view taken along the line A-A of FIG. 3. This neutron lens includes a body part 20, and upper and lower annular outer frames 21 and 22 that fix the body part 20. The body part 20 is sandwiched between the upper and lower annular outer frames 21 and 22, and the outer frames 21 and 22 are fixed on pins 23 arranged between the outer frames 21 and 22 by screws 24 so that the neutron lens can be assembled.
FIGS. 5A and 5B show a structure of a plate member 25 that constitutes the body part 20. To assemble the body part 20, many plate members 25 that each have a hole 32 at the center thereof are multi-layered. The plate member at the higher position has the larger hole at the center thereof, and the plate member at the bottom position does not have the hole at the center thereof. Accordingly, the body part 20 has cone-shaped hollow at the center. In the example of FIG. 4, the body part 20 is constituted by 33 plate members 25 that are multi-layered. The reference numerals 33a through 33d designate holes for pins 23.
In FIGS. 5A and 5B, annular protrusions 31 of which sections are triangle-shaped are formed successively on a thin plate in the radial direction of the thin plate to configure the plate member 25. An inclining surface 31a of the annular protrusion 31 has a triangle-shaped section, provides an incident surface inclined with respect to a beam axis of the incident neutron beam, and faces the inside of the concentric circles, that is, the center axis of the neutron lens.
The neutron beam that enters the neutron lens shown in FIGS. 4, 5A and 5B in parallel with the axis of the neutron lens obliquely enters the inclining surface of the annular protrusion 31 formed on the plate member. For this reason, the neutron beam is deflected toward the center axis of the neutron lens. A part of the neutron beam that enters the center part of the neutron lens penetrates through the relatively small number of the annular protrusions to be deflected by a small angle. On the other hand, a part of the neutron beam that enters the peripheral part of the neutron lens penetrates through the relatively large number of the annular protrusions to be deflected by a large angle. As a result, the neutron lens performs a function similar to that of a convex lens in an optical system, and thus, can focus the neutron beam on a minute region.
Contrary to the example of FIG. 5, if the inclining surfaces 31a of the annular protrusions 31 are formed to face the outer side of the concentric circles, the neutron lens can perform a function similar to that of a concave lens, and can force the neutron beam to diverge with the same configuration as that of FIG. 4.
As described above, the plate member 25 need be made of a material that has a refraction index of less than 1 for a neutron beam. This material includes oxygen O, carbon C, beryllium Be and fluorine F among those having naturally occurring isotopic concentrations, and deuterium D among enriched isotopes. Specifically, the material of the plate member 25 is polytetrafluoroethylene. (PTFE), quartz, MgF2, lead glass, glassy carbon, polyethylene deuteride formed by replacing hydrogen of polyethylene with deuterium, or the like.
Among these materials, quartz, MgF2, lead glass, and glassy carbon (hereinbelow, simply referred to as carbon) are relatively easily available, and desirably, the plate member is formed from the carbon plate.
However, the carbon is hard and fragile, so that the edge part of the annular protrusion is broken by usual machining such as cutting. For this reason, there is a problem in that the material cannot be machined to have a desired shape. In other words, since it is necessary to form the neutron lens by multi-layering many plate members 25, the thinner plate member 25 is better to downsize the neutron lens. For example, desirably, the plate member 25 is about 1 mm in thickness. However, if carbon plate is made thin, the carbon plate is broken by a slight machining resistance. Furthermore, to accurately deflect the neutron beam, it is necessary to raise accuracy of the inclining surface 31a of the annular protrusion 31. In addition, to increase penetration efficiency of the neutron beam while suppressing diffused reflection of the neutron beam on the surface of the neutron lens, the inclining surface 31a need be finished to have a surface roughness near a mirror surface.
In order to solve the above problems, the inventor of the present invention et al devised a method for machining a neutron lens and filed a patent application of this method (refer to Japanese Laid-Open Patent Publication No. 2001-062691). According to this method, as schematically shown in FIG. 6, one or more tapered surfaces 33a of a grinding wheel makes with each other an angle that is sharper than an angle of a V-shaped groove formed on the surface of the neutron lens member 32. The grinding wheel 33 is positioned by a grinding wheel driving machine 34 such that the axis of the grinding wheel 33 is tilted from the rotational axis of the neutron lens member 32. At this position, the tilting angle of the axis of the grinding wheel is changed such that the grinding wheel slightly swings.
However, in this machining method, it is difficult to avoid change of a sectional shape of the tool caused by frictional wear. As a result, sectional shapes of the minute grooves are changed, and thereby, it also becomes difficult to control a surface roughness of the optical surface of the device. Consequently, neutron beam controlling performance of the device is lowered, a cost for correcting the changed shape of the tool is required, and machining efficiency is deteriorated.